Notice that the method name has been translated into XML (the rules by which
a name in a language like Java gets turned into an XML name, and vice versa, can
be found in part 2 of the SOAP 1.2 spec), and we've put it in a namespace
that is specific to our service (this is common practice, but not strictly
necessary). The method invocation, as we mentioned, is an encoded struct with
one accessor for each argument, so the arg element is inside the
method element, and the argument contains the value we're passing: 33.

If we pass this message to a service, the response looks something like
this:

The RPC response is also modeled as a struct, and by convention the name of
the response struct is the name of the method with Response appended to
the end. The struct contains accessors for all inout and out
parameters in the method call (see the next section) as well as the return
value.

The first accessor in the struct is interesting, and it brings to light
another difference between SOAP 1.1 and 1.2: In SOAP 1.1's RPC style, there
was no way to tell which accessor in the struct was the return value of the
method and which were the out parameters. This was a problem unless you
had good meta-data, and even then the situation could be confusing. SOAP 1.2
resolves this issue by specifying that an RPC response structure containing a
return value must contain an accessor named result in the SOAP RPC
namespace. The value of this field is a QName that names the accessor containing
the return value for the invocation.

out and inout Parameters

In some environments, programmers use out parameters g to enable
returning multiple values from a given RPC call. For instance, if we wanted to
return not only a Boolean yes/no value from our inventory check service but also
the actual number of units in stock, we might change our signature to something
like this (using pseudocode):

boolean doCheck(String SKU,
int quantity,
out int numInStock)

The idea is that the numInStock value is filled in by the service
response as well as returning a Boolean true/false. Inout parameters g
are similar, except that they also get passed inso we could use an
inout like this:

boolean doCheck(String SKU,
inout int quantity)

In this situation, we'd pass a quantity value in, and then we expect the
value of the quantity variable to have been updated to the actual
quantity available by the service.

Java developers aren't used to the concept of inout or
out parameters because, typically, in Java all objects are
automatically passed by reference. When you're using RMI, simple objects
may be passed by value, but other objects are still passed by reference. In this
sense, any mutable objects (whose state can be modified) are automatically
treated as inout parameters. If a method changes them, the changes are
seen automatically by anyone else.

In Web services, the situation is different: All parameters are passed by
value. SOAP has no notion of passing parameters by reference. This design
decision was made in order to keep SOAP and its data encoding simple. Passing
values by reference in a distributed system requires distributed garbage
collection and (potentially) a lot of network round-trips. This not only
complicates the design of the system but also imposes restrictions on some
possible system architectures and interaction patterns. For example, how can you
do distributed garbage collection when the requestor and the provider of a
service can both be offline at the same time?

Therefore, for Web services, the notion of inout and out
parameters doesn't involve passing objects by reference and letting the
target backend modify their state; instead, copies of the data are exchanged.
It's then up to the service client code to create the perception that the
state of the object that has been passed in to the client method has been
modified. We'll show you what we mean. Let's take the modified
doCheck() you saw earlier:

boolean doCheck(String SKU,
inout int quantity)

When this method is called, the request message looks like this on the
wire:

This is a request to see if 3 items are available, and the response indicates
that not only are there 3 (the true response), but there are in fact 72 (the new
quantity value). The endpoint receiving this response should then update the
appropriate programming-language construct for the quantity parameter
with the new value.

Finally, here's the example that adds an extra out parameter
containing the number of items in stock to our doCheck() method:

boolean doCheck(in sku, in quantity, out numInStock)

If we called this new method, the request would look identical to the one you
saw earlier in this chapter, but the response would now look like this: